"The national security risks of compromising a smartphone used by a senior government official, such as the President of the United States, are considerable," they wrote in the letter.

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They added that "hackers are sometimes successful even when smartphone users take recommended precautions and restrict access to their personal information. These vulnerabilities are among the reasons why national security agencies discourage the use of personal devices."

McCaskill and Carper also raised concerns that Trump's tweets aren't being properly archived if he's sending them from a personal phone.

The New York Times reported late last month that the president still had an "old, unsecured Android phone, to the protests of some of his aides."

The two Democratic senators are asking the Defense Information System Agency (DISA), which is under the Pentagon's umbrella, to provide a written response on whether Trump was given a secured, encrypted smartphone for personal use before or since the inauguration and a confirmation on whether or not Trump is using the phone.

If he's not, the two senators want to know what kind of phone the president is using.

They also want details on what steps DISA has taken, or will take, to protect the president's personal smartphone and if they coordinated with the Secret Service and National Security Agency on the guidelines or consulted the National Archives and Records Administration.

McCaskill and Carper are asking Mattis to provide the details by March 9.